Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth
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Abstract:
K-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87Sr/86Sr ratios (ISr) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high ISr–Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of ISr, but a larger range of Rb/Sr. Lower ISr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high ISr silicic melts (as seen in the inner cores) and (2) later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.Keywords:
Igneous differentiation
Petrogenesis
Magma chamber
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Fractional crystallization (geology)
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Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.
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Detailed study of the texture and chemical zoning of phenocrysts in a plinian fall deposit (Yunokuchi Pumice) from Akagi Volcano, Japan, permits reconstruction of the subvolcanic magma chamber. Hypersthene phenocrysts are complexly zoned; some have spongy cellular textures whereas others exhibit simple reverse zoning. Augite phenocrysts have texturally similar cores but Al and Ti vs mg-number zoning patterns are correlatable with hypersthene cores. Most plagioclase phenocrysts have a uniform calcic core enclosed by a normally zoned rim; others have variable An contents and mottled textures. These characteristics and melt (glass) compositions suggest a complex, multistage mixing of lot-T (940–960°C) highly silicic and high-T (980–1060°C) less silicic magmas. Stratigraphic variations of the phenocryst types show that the earlier erupted magma inherited phenocrysts from more high-T magma components. We propose the presence of an inhomogeneous mushy chamber filled with a highly silicic magma which was recharged periodically by less silicic magmas. The large viscosity contrast and density balance of the new magma relative to the mush controlled the efficiency of mixing and the position of emplacement of the high-T magma in the mush. Tapping the chamber first expelled the fluidal mixed magma. Followed by progressive erosion of the mush as the eruption continued.
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